1. Field of the Invention
This invention relates to thermal storage systems and more particularly to an ice storage system of the kind used for the mechanical cooling of office buildings, theaters, churches and the like, such storage systems being commonly known as ice builders. By operating such an ice storage system to produce a storage of refrigeration in the form of ice during off peak electrical loads, a reduction in the cost of electrical energy may be achieved. Furthermore, the size of the refrigeration equipment which would otherwise be required for meeting peak demands may be reduced.
2. Description of the Prior Art
The concept of thermal storage by means of ice has been in use for a long time. In the dairy industry, which requires a large heat removal to cool milk in a short period of time, ice has been used to transfer heat from the milk into the cold water side of a heat exchanger, the water returning to the tank and being re-cooled by ice melting. This concept has carried over and is now in substantial use in the air conditioning industry.
The U.S. Pat. No. 3,484,805 to Lorenz discloses an ice bank builder having an ice thickness sensor for controlling the supply of refrigerant to the ice bank.
The U.S. Pat. No. 2,246,401 to Waterfill et al. discloses a refrigeration system in which high side liquid is supplied to a lower ice builder coil and to an upper water chilling coil.
The U.S. Pat. No. 2,722,108 to Hailey discloses an ice builder which is supplied by refrigerant from the high side of a refrigeration system and having an ice thickness detector for controlling the flow of refrigerant thereto.
The U.S. Pat. No. 3,653,221 to Angus discloses an ice builder having a plurality of separate coils arranged side by side and with separate control circuits therefor, supplied from the high side of a refrigeration system.
The U.S. Pat. No. 2,503,212 to Patterson discloses a plurality of cooling coils, each mounted on a separate floor of a building, the coils being supplied with refrigerant from a low pressure receiver, the vapor from the coils being returned thereto, and having individual control means on the respective return lines.
The U.S. Pat. No. 1,954,695 to Garland discloses a refrigerant circuit in which a low pressure receiver or accumulator supplies refrigerant by gravity flow to a coil, the refrigerant vapor being returned to the low side receiver.
The U.S. Pat. No. 2,039,796 to Hiller discloses chilling apparatus in which liquid refrigerant is pumped from a low side receiver or accumulator to a series of coils, the vapor then returning to the low side receiver.
The U.S. Pat. Nos. 1,823,106, to King, 1,869,917, to Schmieding and 2,021,052 to Dickey disclose refrigeration systems having plural evaporator sections for producing different temperature levels, the evaporator sections being connected to a common high side of a refrigeration system.
The U.S. Pat. No. 2,308,079 to Henney discloses in FIG. 2 an evaporator having parallel sections connected to the high side of a refrigeration system for the purpose of meeting different refrigeration requirements.
The U.S. Pat. No. 2,249,856 to Ruff discloses an air conditioning system having a pair of evaporator coils connected in parallel in a cooling duct, the coils being connected to the high side of a refrigeration system.
The 1987 Ashrae Handbook for Heating, Ventillating, and Air Conditioning Systems and Applications, at pages 46.22 and 46.23, discloses an ice builder including a pair of coils mounted in parallel, the coils being positioned in separate tanks, and connected to a low pressure receiver, each coil having a solenoid control valve between the coil and the low pressure receiver for controlling the flow of liquid refrigerant thereto.
In an ice building coil for thermal storage purposes, sufficient ice must be formed to provide the necessary storage of energy. However, if the ice bridges over from one coil to the other, this interferes with the free flow of heat transfer liquid through the ice and therefore impedes the transfer of energy. Attempts have been made to meet this problem by providing an ice sensor at appropriate locations on the coil and shutting off the flow of refrigerant when the ice thickness reaches a predetermined amount.
Furthermore, in some systems, such as that disclosed in the 1987 Ashrae publication, referred to above, the feed of refrigerant has been from the top of the coil to the bottom, on the theory that the tendency of the ice to build from the bottom of a coil will be overcome. However, the problem with this and other systems has been that the liquid refrigerant in the coil flows to the bottom during normal operation; furthermore, when the flow is shut off, any liquid remaining in the coil gravitates to the bottom thereof and in a sufficient amount to continue ice building, resulting in ice bridging.
Thus, regardless of the type of refrigerant feed, whether from top to bottom, bottom to top, gravity flooded or thermal expansion valve control, in vertical serpentine coils there has always been the problem of greater ice thickness at the bottom pipes than those at the top pipes. The result has been that in all such coil systems, the ice builds first on the bottom pipes of a coil and very often such ice bridges over the coils and instead of individual ice covered pipes, there exists a massive block of ice in the bottom area of the tank. Such ice building greatly reduces the exposed surface of the ice and therefore prevents a fast melt down when maximum cooling is required from the recirculated heat transfer water.